If the comment message includes non-English characters, using method provided by user456814, those characters will be replaced by question marks. (tested under sourcetree Ver2.5.5.0)
So I have to use the following method.
CAUTION: if the commit has been pulled by other members, changes below might cause chaos for them.
Step1: In the sourcetree main window, locate your repo tab, and click the "terminal" button to open the git command console.
Step2:
[Situation A]: target commit is the latest one.
1) In the git command console, input
git commit --amend -m "new comment message"
2) If the target commit has been pushed to remote, you have to push again by force. In the git command console, input
git push --force
[Situation B]: target commit is not the latest one.
1) In the git command console, input
git rebase -i HEAD~n
It is to squash the latest n commits. e.g. if you want to edit the message before the last one, n is 2.
This command will open a vi window, the first word of each line is "pick", and you change the "pick" to "reword" for the line you want to edit. Then, input :wq to save&quit that vi window. Now, a new vi window will be open, in this window you input your new message. Also use :wq to save&quit.
2) If the target commit has been pushed to remote, you have to push again by force. In the git command console, input
git push --force
Finally: In the sourcetree main window, Press F5 to refresh.